The present invention relates to a method for converting a real signal into a complex signal, so that the in-phase and quadrature components are inherently in quadrature.
It is known that real/complex converters, also known as IQ demodulators, used to convert a pass-band signal into a baseband signal in order to allow its subsequent digital processing, and synchronous receivers, also known as ZERO-IF receivers, use a conversion of a real signal into a complex signal.
This process of conversion from pass-band to baseband is achieved by multiplying the input signal with a complex carrier whose frequency is equal to the center frequency.
In this regard, FIG. 1 illustrates the input signal, designated by the reference numeral 1, which is introduced in multiplier means 2 and 3 being designed to respectively multiply the input signal 1 with a complex carrier whose frequency is equal to the center frequency.
The reference numeral 4 designates an oscillator whose frequency is equal to the center frequency and the reference numerals 5 and 6 respectively designate the complex component (sine) and the real component (cosine) of the carrier. The input signal 1 is then multiplied in the multiplier 2 with the imaginary or complex component 5 of the carrier (sine component) and, in the multiplier 3, with the real component 6 of the carrier (cosine).
However, the generation of the real component (cosine) and imaginary component (sine) of the carrier, although conceptually simple, raises great problems in terms of implementation.
In particular, it is almost impossible to achieve high precision in the phase ratio, which should be 90 degrees.
In practice, errors of even one or two degrees are absolutely common, when instead a good conversion should contain the errors within a few millionths of a degree.
Phase errors in conversion produce important distortions of the signal and/or of the information carried by the signal, as in the case of FM modulation and ZERO-IF receivers.